The flap is a reinforcement for the rear quarter of the top, and it is normally fastened with some contact adhesive when the top is manufactured. The real fastener for this flap is staples at the top and bottom. It appears that the top of the flap has come loose from the staples. This could be for a couple of reasons: the top of the flap has deteriorated and torn away, or the staples holding the top of the flap have rusted away.
The solution is to fold back the bead that goes across the back of the top and remove enough of the staples so the top can be lifted up from the frame. This can be achieved by putting the top halfway down so the vinyl is loose by the back bow.
If the flap is still intact, glue it to the top so it stays in place. If there are areas of the top of the flap that are missing due to rot or tearing, make a note of these spots. Then lay the top with the flap attached back down onto the rear bow and put the top up so the vinyl is taut over the back bow.
Noting where the good vinyl is on the flap, staple the top and flap back in place. Then reattach the folding hidem strip and fold over to hide the staples.
Good luck. Feel free to call me if you get stuck. 360-574-8070